This invention relates generally to a method and system for reference normalization in an x-ray image, and more particularly, to a method and system for reference normalization for blocked reference channels in an x-ray image generated by a computed tomography (CT) system.
In CT imaging systems, an x-ray source projects a fan-shaped beam that is collimated to lie within an X-Y plane, generally referred to as an “imaging plane”, of a Cartesian coordinate system toward an array of radiation detectors, wherein each radiation detector includes a detector element disposed within the CT system so as to receive this fan-shaped beam. An object, such as a patient, is disposed between the x-ray source and the radiation detector array so as to lie within the imaging plane and so as to be subjected to the x-ray beam, which passes through the object. As the x-ray beam passes through the object, the x-ray beam becomes attenuated before impinging upon the array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is responsive to the attenuation of the x-ray beam by the object, wherein each detector element produces a separate electrical signal responsive to the beam intensity at the detector element location. These electrical signals are referred to as x-ray attenuation measurements or x-ray images.
Moreover, the x-ray source and the detector array may be rotated, with a gantry within the imaging plane, around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and the detector array. In an axial scan, the projection data is processed so as to construct an image that corresponds to a two-dimensional slice taken through the object. In CT systems that employ a single detector array, the slice thickness is controlled and determined by the width of the collimator, while in CT systems that employ a multiple detector array, the slice thickness is controlled and determined by summing the contributions of a plurality of detector sub-units and by physically moving the collimator to the outer edges of each slice.
Reference normalization is an important step in CT preprocessing operations to reduce or remove the impact of the x-ray tube output fluctuation. For this purpose, a set of reference channels (also referred to herein as reference detectors) are placed slightly outside the reconstruction field of view (FOV) in the array of radiation detectors so that the reference channels receive x-ray photons directly from the x-ray source without interference from the scanned object. These reference channels monitor the x-ray tube flux and the measured signal is applied to the measured projections. The impact of any variations in the x-ray tube output on the measured projections is thereby properly removed. In a typical configuration, three reference channels are placed at the left-hand side of the array. However, for larger than normal-sized patients and/or with the scanned object off-centered, there is a greater chance that these reference channels are blocked during the scan. In addition, with the introduction of wide-bore scanners that cover a larger FOV with the same detector, the reference channels are routinely blocked by the patients. To ensure the proper functioning of the reference normalization, different correction steps have to be developed to account for situations where the reference channels are blocked.